This invention relates to a holographic projection device with an array of mirror elements. This invention further relates to a method for the enlargement of a reconstruction volume for viewing a reconstructed, preferably three-dimensional scene, where an illumination device with at least one light source emits sufficiently coherent light.
Holography allows three-dimensional recording and optical representation of objects using wave-optical methods. The reconstruction of the holographic image, often referred to as reconstruction, is realised, depending on the type of hologram, by illuminating a hologram-bearing medium with coherent light. In prior art holographic projection devices the reconstruction volume, or viewing angle, is too small for viewing in particular a three-dimensional scene.
Usually, the reconstruction is viewed directly, i.e. an observer looks on to a computer-generated hologram (CGH), which consists of regularly arranged pixels which are to be encoded in accordance with hologram values. Due to effects of deflection, the reconstruction of the CGH is only achievable within one periodicity interval, which is defined by the resolution of the CGH. The reconstruction is typically repeated showing irregularities in adjacent periodicity intervals. The size of the region to be represented is thus limited by the resolution. The resolution of the hologram would have to be increased substantially in order to enlarge the viewing angle at least to an extent that the scene can be watched with both eyes.
The aim of an extended reconstruction volume and large viewing angle for the holographic reconstruction of an object thus requires a hologram-bearing medium which has a large number of minute pixels. The pixels should be as small as possible and their optical properties should be discretely controllable. Such small distances between the pixels (pitches), which describe the resolution, require costly manufacturing processes for the arrays.
Recording media for CGHs include light modulators, such as LCD, LCoS, acousto-optic modulators, OASLM and EASLM, which modulate the phase and amplitude of incident light.
WO 2005/059659 A2 describes for example a device with a light modulator for the enlargement of the viewing angle in holographic displays. A phase mask with a resolution greater than that of a light modulator used for representing the hologram is disposed immediately behind the light modulator, seen in the direction of light propagation. Each pixel of the light modulator is associated with four or more elements of the phase mask. The phase mask thus generates a higher virtual resolution, and thus an enlarged viewing angle.
However, these benefits are at the cost of additional noise, because the phase mask is the same for each object and there is a random distribution of values when increasing the resolution.
Moreover, light modulators are known which comprise micro-mirrors for light modulation. Such light modulators are used to modulate the amplitude and/or phase of incident light.
U.S. Pat. No. 6,028,689 describes a micro-mirror which is suspended on four carrier arms of a holding bracket. The micro-mirror can be moved along two axes by supplying a voltage to the electrodes provided. The micro-mirror is displaced axially in order to reduce or at least to minimise phase errors in the image.
Document CA 2 190 329 C describes a light modulator for the modulation of the amplitude and phase of incident light. The light modulator comprises micro-mirrors, and between a base plate of the light modulator and each micro-mirror there is a flexural element which will tilt or axially displace the micro-mirror relative to the base plate if an electrostatic force is applied. For amplitude modulation, the micro-mirror of the light modulator is tilted by supplying a voltage between the micro-mirror and an electrode on the base plate of the light modulator. If a voltage is simultaneously supplied to two electrodes on the base plate, the electrostatic force causes an axial movement of the micro-mirror, thus effecting phase modulation.
The reconstruction volume and thus the viewing angle can only be enlarged using the light modulator known from CA 2 190 329 C by increasing the number of pixels and thus by improving the resolution. Moreover, the light modulator described in the aforementioned document is applied to relatively large mirrors (>50 μm).